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Taikun OCP Guide

Table of Contents

AMD SEV (Secure Encrypted Virtualization)

20.0.0 (Train)

Secure Encrypted
Virtualization (SEV) is a technology from AMD which enables the
memory for a VM to be encrypted with a key unique to the VM. SEV is
particularly applicable to cloud computing since it can reduce the
amount of trust VMs need to place in the hypervisor and administrator of
their host system.

Enabling SEV

First the operator will need to ensure the following prerequisites
are met:

  • Currently SEV is only supported when using the libvirt compute
    driver with a libvirt.virt_type of kvm
    or qemu.
  • At least one of the Nova compute hosts must be AMD hardware capable
    of supporting SEV. It is entirely possible for the compute plane to be a
    mix of hardware which can and cannot support SEV, although as per the
    section on Permanent limitations
    below, the maximum number of simultaneously running guests with SEV will
    be limited by the quantity and quality of SEV-capable hardware
    available.

In order for users to be able to use SEV, the operator will need to
perform the following steps:

Additionally the cloud operator should consider the following
optional steps:

  • Configure the libvirt.num_memory_encrypted_guests
    option in nova.conf to
    represent the number of guests an SEV compute node can host concurrently
    with memory encrypted at the hardware level. For example:

    [libvirt]
num_memory_encrypted_guests = 15

    This option exists because on AMD SEV-capable hardware, the memory
    controller has a fixed number of slots for holding encryption keys, one
    per guest. For example, at the time of writing, earlier generations of
    hardware only have 15 slots, thereby limiting the number of SEV guests
    which can be run concurrently to 15. Nova needs to track how many slots
    are available and used in order to avoid attempting to exceed that limit
    in the hardware.

    At the time of writing (September 2019), work is in progress to allow
    QEMU and libvirt to expose the number of slots available on SEV
    hardware; however until this is finished and released, it will not be
    possible for Nova to programmatically detect the correct value.

    So this configuration option serves as a stop-gap, allowing the cloud
    operator the option of providing this value manually. It may later be
    demoted to a fallback value for cases where the limit cannot be detected
    programmatically, or even removed altogether when Nova’s minimum QEMU
    version guarantees that it can always be detected.

    Note

    When deciding whether to use the default of None or
    manually impose a limit, operators should carefully weigh the benefits
    vs. the risk. The benefits of using the default are a) immediate
    convenience since nothing needs to be done now, and b) convenience later
    when upgrading compute hosts to future versions of Nova, since again
    nothing will need to be done for the correct limit to be automatically
    imposed. However the risk is that until auto-detection is implemented,
    users may be able to attempt to launch guests with encrypted memory on
    hosts which have already reached the maximum number of guests
    simultaneously running with encrypted memory. This risk may be mitigated
    by other limitations which operators can impose, for example if the
    smallest RAM footprint of any flavor imposes a maximum number of
    simultaneously running guests which is less than or equal to the SEV
    limit.

  • Configure ram_allocation_ratio on all SEV-capable
    compute hosts to 1.0. Use of SEV requires locking guest
    memory, meaning it is not possible to overcommit host memory.

    Alternatively, you can explicitly configure small pages for instances
    using the hw:mem_page_size flavor extra spec and
    equivalent image metadata property. For more information, see huge-pages.

  • Configure libvirt.hw_machine_type on all
    SEV-capable compute hosts to include x86_64=q35, so that
    all x86_64 images use the q35 machine type by default.
    (Currently Nova defaults to the pc machine type for the
    x86_64 architecture, although it is
    expected that this will change in the future    __.)

    Changing the default from pc to q35 makes
    the creation and configuration of images by users more convenient by
    removing the need for the hw_machine_type property to be
    set to q35 on every image for which SEV booting is
    desired.

    Caution

    Consider carefully whether to set this option. It is particularly
    important since a limitation of the implementation prevents the user
    from receiving an error message with a helpful explanation if they try
    to boot an SEV guest when neither this configuration option nor the
    image property are set to select a q35 machine type.

    On the other hand, setting it to q35 may have other
    undesirable side-effects on other images which were expecting to be
    booted with pc, so it is suggested to set it on a single
    compute node or aggregate, and perform careful testing of typical images
    before rolling out the setting to all SEV-capable compute hosts.

    __ https://bugs.launchpad.net/nova/+bug/1780138

Configuring a flavor or
image

Once an operator has covered the above steps, users can launch SEV
instances either by requesting a flavor for which the operator set the
hw:mem_encryption extra spec to
True, or by using an image with the
hw_mem_encryption property set to True. For
example, to enable SEV for a flavor:

$ openstack flavor set FLAVOR-NAME \
    --property hw:mem_encryption=true

These do not inherently cause a preference for SEV-capable hardware,
but for now SEV is the only way of fulfilling the requirement for memory
encryption. However in the future, support for other hardware-level
guest memory encryption technology such as Intel MKTME may be added. If
a guest specifically needs to be booted using SEV rather than any other
memory encryption technology, it is possible to ensure this by setting
the trait{group}:HW_CPU_X86_AMD_SEV extra spec
or equivalent image metadata property to required.

In all cases, SEV instances can only be booted from images which have
the hw_firmware_type property set to uefi, and
only when the machine type is set to q35. This can be set
per image by setting the image property
hw_machine_type=q35, or per compute node by the operator
via libvirt.hw_machine_type as explained
above.

Limitations

Impermanent limitations

The following limitations may be removed in the future as the
hardware, firmware, and various layers of software receive new
features:

  • SEV-encrypted VMs cannot yet be live-migrated or suspended,
    therefore they will need to be fully shut down before migrating off an
    SEV host, e.g. if maintenance is required on the host.

  • SEV-encrypted VMs cannot contain directly accessible host devices
    (PCI passthrough). So for example mdev vGPU support will not currently
    work. However technologies based on vhost-user__ should work fine.

    __ https://wiki.qemu.org/Features/VirtioVhostUser

  • The boot disk of SEV-encrypted VMs can only be
    virtio. (virtio-blk is typically the default
    for libvirt disks on x86, but can also be explicitly set e.g. via the
    image property hw_disk_bus=virtio). Valid alternatives for
    the disk include using hw_disk_bus=scsi with
    hw_scsi_model=virtio-scsi , or
    hw_disk_bus=sata.

  • QEMU and libvirt cannot yet expose the number of slots available
    for encrypted guests in the memory controller on SEV hardware. Until
    this is implemented, it is not possible for Nova to programmatically
    detect the correct value. As a short-term workaround, operators can
    optionally manually specify the upper limit of SEV guests for each
    compute host, via the new libvirt.num_memory_encrypted_guests
    configuration option described above
    <num_memory_encrypted_guests>    .

Permanent limitations

The following limitations are expected long-term:

  • The number of SEV guests allowed to run concurrently will always
    be limited. On the first generation of EPYC
    machines it will be limited to 15 guests    __; however this limit
    becomes much higher with the second generation (Rome).

    __ https://www.redhat.com/archives/libvir-list/2019-January/msg00652.html

  • The operating system running in an encrypted virtual machine must
    contain SEV support.

Non-limitations

For the sake of eliminating any doubt, the following actions are
not expected to be limited when SEV encryption is used:

  • Cold migration or shelve, since they power off the VM before the
    operation at which point there is no encrypted memory (although this
    could change since there is work underway to add support for PMEM)
  • Snapshot, since it only snapshots the disk
  • nova evacuate (despite the name, more akin to
    resurrection than evacuation), since this is only initiated when the VM
    is no longer running
  • Attaching any volumes, as long as they do not require attaching via
    an IDE bus
  • Use of spice / VNC / serial / RDP consoles
  • VM guest virtual NUMA <cpu-topologies>

References

  • libvirt driver launching AMD SEV-encrypted
    instances (spec)    __

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